This invention relates to oil burners. In particular this invention relates to a method of and apparatus for preheating oil for combustion, e.g. preheating waste oil for combustion in a waste oil burner. The inventive method and apparatus are uniquely effective in precluding nozzle drip.
The onset of increased environmental awareness heralded concerns for developing new and improved methods for disposing of waste products. High on the list of waste products to be soundly disposed of has been waste oil. The term waste oil has been used to define heavy oil as well as spent or used oil. Heavy oils include API designations Nos. 5 and 6. These oils are difficult to combust in view of their complex hydrocarbon structure and the fact that they ordinarily contain impurities at inconsistent levels. Waste oils such as used cutting oil, used automotive oils and the like, exhibit many of the combustion characteristics of heavy oils. Further, their characteristics are changed by additives, foreign particles, and the fact that through use their viscosity tends to increase.
All these factors have resulted in difficulties being experienced in designing methods and apparatus for the efficient combustion of waste oil. The coking temperature of waste oils is ordinarily much lower than that of corresponding clean oils. Complicating this is the fact that use and the existence of additives and other impurities increase the viscosity of the waste oils thereby requiring a relatively high degree of preheat to achieve proper atomization for combustion.
Approaches which have been advanced in attempting to deal with these and other such problems are shown in U.S. Pat. Nos. 4,249,885, 4,406,943 and 4,797,089. However, these methods, for example those such as that disclosed in U.S. Pat. No. 4,797,089, have experienced difficulties, particularly in applications involving start-stop operations such as those required in the operation of space heaters or unit heaters.
It is well known to those skilled in these arts that in waste oil heater applications involving start-stop or on-off operations, nozzle drip has been a continuing problem. In order to achieve the temperature needed to properly atomize the waste oil, preheaters have been positioned substantially adjacent the burner nozzles and sized such as to be able to transfer sufficient heat to increase the temperature of the flowing oil to atomization temperature. This is fine so long as the system is operating and the oil flowing. However, when the thermostat controlling the space heater calls for the space heater to turn off, the flow of oil stops. Known preheaters are not capable of instantaneous termination of heat transfer to the waste oil. More specifically, known preheaters act as "heat sinks" and transfer heat to waste oil in the preheater even after flow terminates. The continued transfer of heat to the waste oil causes the temperature of the waste oil to increase above that which would otherwise be satisfactory to atomize the waste oil for combustion.
In extreme situations such temperature increases can cause coking of the waste oil with resultant clogging of the waste oil supply line. What occurs more usually, however, is that the oil will expand. Because back flow of the oil in response to such expansion ordinarily is precluded by a positive block in the oil line established either by a valve or a positive-displacement waste oil pump, the hot expanded waste oil follows the path of least resistance and flows out of the burner nozzle into the combustion chamber. This is "nozzle drip". The adverse effects of such "nozzle drip" are well known.